Hyundai Motor Group, the world's fifth-largest automotive conglomerate, said Wednesday that its U.S. sales moved up 3.7 percent in March, while its hold on the domestic market slipped slightly.

The conglomerate said combined sales of cars made by Hyundai Motor Co. and affiliate Kia Motors Corp. in the United States stood at 121,782 units last month, up from 117,431 tallied a year earlier. The total is equivalent to 17.3 percent of all 703,534 cars sold worldwide by the two companies during the month.

It said Kia sales rose by double digits to push up overall sales numbers and offset a dip in demand for Hyundai-made cars.

Kia Motors America, Kia's distribution and marketing arm in the U.S., reported that sales ballooned 11.5 percent last month to 54,777 units, with demand for the Optima midsize sedan and Soul boxy-compact leading gains.

Monthly sales of the popular Optima hit 16,310 units, up 13.5 percent on-year. This is a monthly record for the model. Sales of the second-generation Soul jumped 15.8 percent to a record 13,992 vehicles, KMA said.

It also said the K900 luxury sedan that went on sale for the first time last month sold 105 units, which is good for a "low volume" car that competes with top of the line German and Japanese cars sold in the United States.

Hyundai Motor America, on the other hand, reported sales dipping 1.9 percent to 67,005 units last month, as demand for its best-selling Elantra plunged 17.7 percent vis-a-vis March 2013 to 21,518 units.

The company said despite Elantra's poor showing, sales of its refreshed 2014 Sonata hit 19,248 units for a gain of 6.9 percent, with numbers for the Santa Fe sport utility vehicle soaring 36.7 percent to 9,726.

Despite general gains in the U.S., market share of Hyundai and Kia in South Korea fell under the 80 percent mark in March.

Hyundai sold 57,812 cars for a market share of 47.6 percent, while Kia reported selling 39,005 units at home for 32.1 percent control of the market. Combined, the two carmakers accounted for 79.7 percent of the 121,416 vehicles sold last month.

Hyundai and Kia had enjoyed market share of 80.4 percent and 80.7 percent in January and February, respectively, after falling to an annual low of 73.8 percent in December of last year.